1. Field of Use
This invention relates generally to wavelength selective filtering of light. More specifically, this invention concerns holographic filters used in spectroscopic and spectral splitting applications.
2. Description of the Prior Art
Devices used to filter out or inhibit the transmission of certain wavelengths of light are in widespread use, and have several particularly important applications. One important application is that of spectrophotometers which are able to determine the chemical composition of a material by illuminating it with a broad wavelength range of light waves. In such devices, light is generated by a source and dispersed and collimated by an imaging system. A slit is moved within the system which determines the wavelength of light to be measured.
The quality of a spectroscopic device depends primarily on two factors. Resolution of the device determines the width of the spectral line that the spectrophotometer will be able to discern. Resolution is directly dependent upon the size of the slit which is usually better than one nanometer in high quality devices. The maximum optical density ("O.D.") is another critical quality parameter of spectroscopic devices. Spectrophotometers of good quality are able to measure O.D.'s of up to 4.0 and sometimes more. O.D.'s of up to 4.0 are difficult to measure because they represent four orders of magnitude attenuation of the incident light intensity. To measure such a large range of O.D. requires that the noise in the system from all sources be very low. Noise is picked up from every conceivable source including ambient light and the electronics in the spectroscopic system itself.
One type of spectrophotometer is the absorption spectrophotometer wherein the composition of the specimen is determined by measuring the light absorbed by the specimen. A logarithmic scale is used to plot absorption in order to accommodate the several orders of magnitude of O.D. O.D. is represented by the following equation EQU O.D.=-log.sub.10 T
where T represents the transmission coefficient of the material sample. For example, if T equals 10.sup.-4, O.D. equals 4.0 Referring to FIG. 1, O.D. is plotted against wavelength .lambda.. It can be seen that where the curve in FIG. 1A reaches its maximum near the wavelength .lambda..sub.O, the material is optically dense and indicates that the specimen in the spectrophotometer has absorbed the range of wavelengths surrounding .lambda..sub.0. The counterpart to the O.D. versus .lambda. curve is illustrated in FIG. 1B. FIG. 1B plots T, the transmission coefficient against wavelength .lambda.. It can be seen that all wavelengths of the light source are transmitted by the specimen except for the range of wavelengths about the wavelength .lambda..sub.O. Note that the typical measurement of light absorbed by a specimen is a rather complex curve as can be seen in FIG. 1C. Simplified curves such as those in FIGS. 1A and 1B will be used throughout for simplicity.
There exists a certain class of spectroscopy called laser spectroscopy. In laser spectroscopy a laser beam having wavelength .lambda..sub.0 is used rather than some wider source beam. In laser spectroscopy the laser beam is incident upon a scattering medium of interest which, according to its chemical composition, will scatter the laser beam into multiple beams some of which have the same wavelength as the incident beam and some of which have a different scattered wavelength. Referring now to FIG. 2 a laser beam .lambda..sub.0 is incident upon scattering medium S. The incident beam is scattered into a multiplicity of light waves each having a scattered wavelength .lambda..sub.S. It can be seen that the wavelength of some of the light waves .lambda..sub.S is equal to .lambda..sub.0 and the wavelength of other .lambda..sub.S light waves is not equal to .lambda..sub.O. The scattered light waves whose wavelength is .lambda..sub.0 can be said to have undergone elastic scattering. The scattered light waves whose wavelength is not equal to .lambda..sub.O, can be said to have undergone inelastic scattering.
Elastic scattering means that the scattered light photons have the same energy as the incident light photons. Elastic scattering is by far the stronger of the two scattering effects and thus the scattered energy to be measured is usually heavily biased toward the .lambda..sub.s =.lambda..sub.0 light waves. On the other hand, inelastically scattered light photons usually have less energy than the .lambda..sub.0 light photons. The energy of these light photons can be described by the following equation EQU E.sub.p =hf=hc/.lambda.
where E.sub.p is photon energy, h is Planck's constant, f is the frequency in Hertz of the light wave, c is the velocity of light in a vacuum, and .lambda. is the wavelength of the light in a vacuum. Ep for the inelastic scattering case is less than E.sub.p for elastic scattering.
The technological challenge of filtering in laser beam spectroscopy arises from the fact that it is the inelastically scattered wave that contains more information about the chemical structure of the material under test and consequently is the desired signal. Scattered light waves having energy E.sub.P equal to the energy of the .lambda..sub.0 light wave thus constitute noise and must be filtered out, along with the multiplicity of other noise sources so that the desired inelastically scattered signal energy can be measured with accuracy. Filters are needed to block the .lambda..sub.0 wavelength light wave. The most common are Raman filters used for Raman spectroscopy applications.
Wavelength selective optical filters have basically been of two types, absorption dyes and dielectric multilayers. The advantages of absorption dyes as wavelength selective optical filters is primarily their high angular acceptance of nearly .+-.90.degree.. This means that light incident upon the filter at most any angle will be filtered. The disadvantages of absorption dyes stem from the fact that absorption dyes have their own chemical structure and thus their own absorptive characteristics which can affect the absorption measurement from the specimen. Additionally, absorption dyes have rather broad bandwidths and, consequently, have low wavelength selectivity, i.e. bandwidth is usually higher than 20 nm. Furthermore, absorption dyes have secondary maxima due to their sophisticated chemical composition that can be confused with the absorption lines of the specimen. Finally, the disadvantage of an absorption dye stems from what is usually its strong point, broad angular acceptance. This strong point can be a disadvantage where the specimen is tested for angular selectivity.
The second type of known wavelength selective optical filter is the dielectric multilayer. Dielectric multilayer filters operate on the principle of Bragg interference. Bragg interference filters operate on the principle that for certain wavelengths near .lambda..sub.O, the reflected waves interfere constructively with each other and so have a high reflectivity for wavelengths in the vicinity of .lambda..sub.O. For other wavelengths, the reflected waves interfere destructively. Dielectric multilayer filters are usually used as reflection filters. A dielectric multilayer filter is shown in FIG. 3A having alternating dielectric layers made from materials A and B.
There is another type of filter called a Fabry-Perot etalon, however, which is a transmission filter based on interference principles. Fabry-Perot etalon filters transmit some wavelengths and reject all others in contradistinction to reflection type filters which reflect only certain wavelengths and transmit all others. A Fabry-Perot etalon (transmission) filter is shown in FIG. 3B and has a first coating C' comprising dielectric multilayers (not shown) similar to those in FIG. 3A separated from a second coating C" comprising similar alternating dielectric layers. In essence, the Fabry-Perot etalon is a combination of two highly parallel multilayer dielectric coatings and operates by causing interference of the light waves between the two coatings.
A transmission plot for a dielectric multilayer filter is shown in FIG. 4A which illustrates that the filter transmits all wavelengths except those around .lambda..sub.O. A transmission plot for a Fabry-Perot etalon transmission filter is shown in FIG. 4B which illustrates that the filter transmits wavelengths around .lambda..sub.01, .lambda..sub.02, .lambda..sub.03 . . . .lambda..sub.On.
The wavelength selectivity of dielectric multilayer filters is directly dependent upon the number of layers in the filter. The critical importance of this is fully discussed infra. Vacuum deposition is used to produce these filters, by evaporating layer after layer of alternating dielectric materials. Each layer adds to the cost of the filter. Furthermore, the cost of physically larger dielectric multilayer filters becomes prohibitive due to the size of the required vacuum chamber in which the filters are made.
An additional disadvantage of this type of filter is that the rectangular periodic distribution of its refractive index n creates unwanted harmonics and secondary maxima. Certain of the harmonics, particularly the second harmonic, can be suppressed, but other harmonics and secondary maxima remain which can affect the performance of the filter. The secondary maxima, similar to those shown in FIG. 4A, are especially problematic from the standpoint of spectroscopic system accuracy because they can be confused with the absorption spectral lines characterizing the chemical structure of the sample. FIG. 5A shows the variation of the refractive index n for a typical multilayer dielectric filter having alternating dielectric layers A and B. It can be seen that the refractive index n is a rectangular function over the several layers of the filter, and has an average refractive index n, and the grating constant .LAMBDA.. The grating constant can be described by the equation EQU .LAMBDA.=.lambda./2n
Dielectric filters can be made to have a more sinusoidal variation of refractive index n, but the cost of such filters, called rugate filters, is extremely high. FIG. 5B depicts the sinusoidal refractive index profile of a rugate multilayer dielectric filter.
Due to the disadvantages of both the absorption dye and multilayer dielectric filters, and especially the high cost of the latter, there is a need for a filter for use in spectroscopic applications that has extremely high rejection, high wavelength selectivity, high angular acceptance, minimized secondary maxima, and which can be manufactured at low cost.